1. Field of the Invention
The present invention relates to power converters. More particularly, the present invention relates to the control circuit of switching power converters.
2. Background of the Invention
The switching power converter is a traditional technology to control the output power and achieve the regulation. Various protection functions, such as over-voltage and over-current protection are built-in in the power converter to protect the power converter and the connected circuits from permanent damage. The function of output power limit is generally used for the over-load and short circuit protection. Referring to FIG. 1, a traditional switching power converter uses a control circuit 50. The operation of the power converter starts on the charging of a start-up capacitor 65 coupled to the supply terminal VCC of the control circuit 50 via a serial start-up resistor 30 for providing a supply voltage VCC by an input voltage VDC when the power is turned on until the supply voltage VCC is charged up to the threshold voltage, and then the output terminal OUT of the control circuit 50 starts to output a switching signal VPWM and drive the power converter. After the start-up, the supply voltage VCC is provided from the auxiliary bias winding of a transformer 20 through a rectifier 60.
A power transistor 10 is coupled to the primary winding of the transformer 20 and the output terminal OUT. The power transistor 10 switches the transformer 20 to control the output power of the power converter in response to the switching signal VPWM. A resistor 15 that is connected serially with the power transistor 10 determines the maximum output power of the power converter. The method is to connect a resistor 40 to the current-sense terminal VS of the control circuit 50. If the voltage VS is greater than a maximum threshold, the control circuit 50 will disable the switching signal VPWM, and restrict the maximum output power of the power converter. However, the maximum output power is affected by a response time TD. From the moment that the voltage VS in the current-sense terminal VS is detected higher than the maximum threshold to the moment that the switching signal VPWM of the control circuit 50 is actually turned off, there is a delay time TD. The delay time TD causes a different over-power protection in response to the change of the input voltage VDC.
A resistor 35 is added between the input voltage VDC and the current-sense terminal VS for the feedforward compensation. The feedforward compensation is used to compensate the difference of the output power caused by the input voltage VDC and the delay time TD. By properly selecting the resistance of the resistor 35, an identical output power limit for the low line and high line voltage inputs can be obtained. Because the resistors 30 and 35 will cause significant power loss, especially in high line voltage input. Using a resistor for both feedforward compensation and start-up are proposed, it is U.S. Pat. No. 6,611,439 “PWM controller for controlling output power limit of a power supply” by Yang, et al. Moreover, a prior art U.S. Pat. No. 6,906,934 “Integrated start-up circuit with reduced power consumption” by Yang, et al. was developed to further reduce power consumption. However, the skill innovated in U.S. Pat. No. 6,906,934 is inadequate to be applied to the apparatus of U.S. Pat. No. 6,611,439. Therefore, the objective of present invention is to solve the problem of prior arts. In order to save power and reduce device count, a resistor is used for the start-up, feedforward compensation and safety purposes.